In Tremont et al. U.S. Pat. No. 4,749,640, an integrated circuit manufacturing process is disclosed in which ozonated water, containing preferably between 0.02 and 0.09 p.p.m. ozone, is used a part of the manufacturing process. The ozone in the water is a powerful oxidizing agent, for use in conditioning electronic components such as uncovered portions of a photoresist, to provide a beneficial, thin oxide film thereon.
In the prior methods for manufacturing ozonated water used for such purposes, the oxygen content of the water is typically greatly higher than the ozone content. Water initially tends to contain dissolved oxygen and no ozone. Then, pure oxygen gas may be fed through a conventional ozone generator to produce oxygen plus ozone. The resulting oxygen-ozone mixture includes oxygen typically present in a concentration of about 90 percent or more, and ozone present at a concentration of typically less than 10 percent. This mixture may be fed to a compressor, where the gas mixture is compressed to 85 psi, for example, and injected into water through a gas injector.
Thus, by such a process or other typical prior art processes, the oxygen concentration is much greater than the ozone concentration in ozonated water.
It is believed that high oxygen concentrations in ozonated water may interfere with the controlled oxidation of a surface by the ozone present. Thus, it is desirable in at least some circumstances to use ozonated water of lower oxygen content to produce the desired controlled oxidation in electronic components.
It turns out to be difficult to obtain ozonated water with an oxygen content which is reduced to a concentration comparable with the ozone present. Ozone generators generally fail to provide ozone-oxygen mixtures where the two components are in comparable concentrations. Even if they did, such a mixture could be dangerously explosive, and thus undesirable for industrial use because of the potential dangers of the direct addition to water of ozone-oxygen mixtures with an ozone concentration on the order of 50 percent.
Alternatively, while those skilled in the art might address various chemical means for removing oxygen from water, it is not seen how one could effectively and inexpensively remove oxygen from the water without also removing the ozone present, probably at a rate greater than the oxygen removal rate.
In accordance with this invention, a method is provided for preferentially removing oxygen from water which contains a mixture of oxygen and ozone. This method is believed to be the first feasible method for obtaining ozonated water which has a relatively low oxygen content comparable with the ozone content. Such ozonated water is believed to be usable in electronic manufacturing processes to provide thin, high density, adherent oxide films on silicon and other surfaces, with improved results when compared with ozonated water having higher oxygen contents.